A liquid crystal display device (display device) incorporating a non-luminous liquid crystal panel (display panel) typically also incorporates a backlight unit (illumination device) which supplies light to the liquid crystal panel. The backlight unit is so structured as to output planar light with even luminance over the entire area of the planar liquid crystal panel. Some such backlight units are provided with a light guide plate (light guide member) to diffuse the light from the light source widely, thereby to obtain even luminance.
As backlight units including a light guide plate as mentioned above, for example, edge-lit backlight units are known. In an edge-lit backlight unit, typically, a light source is arranged at a side face of the light guide plate. In a so structured backlight unit, the light emitted from the light source enters the light guide plate through a side face thereof. The light is then guided (diffused) inside the light guide plate, and is eventually output, as planar light, toward the liquid crystal panel.
Today, as light sources in backlight units, light-emitting diodes (LEDs) are increasingly used. Compared with conventionally used fluorescent lamps (such as cold-cathode fluorescent lamps), LEDs are themselves more compact, and operate on lower driving voltages; this helps simplify the driving circuit, and allows size and thickness reduction in backlight units. Also, LEDs consume less electric power than cold-cathode fluorescent lamps, and this helps reduce energy consumption (power consumption).
On the other hand, in an edge-lit backlight unit, using a point light source such as an LED as mentioned above often makes it difficult to shine light into the light guide plate evenly over its dimension. This tends to result in, in a backlight unit employing an LED as a light source as mentioned above, bright lines (V-shaped bright lines) with higher luminance appearing in the diffusion pattern of the LED, producing luminance unevenness in the planar light. To suppress such luminance unevenness, for example, JP-A-2002-169034 proposes an illumination device (light guide plate) that outputs even light even in combination with a point light source such as an LED.
JP-A-2002-169034 discloses an illumination device wherein a trapezoidal protrusion is provided in a light guide plate, at a position corresponding to a point light source, and a through hole with a symmetric shape, such as triangular or trapezoidal, is provided in the trapezoidal protrusion. In this illumination device, the light from the light source is reflected on the side faces of the trapezoidal protrusion and on the side faces of the through hole, and thereby the light that has entered the light guide plate is spread laterally. This permits emission of light (planar light) with even luminance.